1. Field of the Invention
The present invention relates to a method for forming photographic images by intensification or amplification, and more particularly, relates to an improvement in an image amplification method using a cobalt (III) complex, that is, relates to a method for increasing the stability of image amplification by using a processing solution containing a cobalt (III) complex and an organic phosphonic acid. 2. Description of the Prior Art
In the field of color photographic methods, a color-forming agent such as a cyan coupler, a yellow coupler or a magenta coupler is incorporated into a silver halide emulsion by various methods, the silver halide emulsion is coated on a support to provide a color photographic element, and after image-wise exposure, the element is subjected to processing to reproduce images on the element.
The processing basically includes a color developing step and a desilvering step. That is, in the color developing step, silver halides of the exposed color photographic element are reduced by a color developing agent to form silver images, and the oxidized color developing agent is reacted with a color-forming agent to form dye images. Then, the color photographic element is subjected to a desilvering step. In this step, the silver images formed in the color developing step are oxidized with an oxidizing agent (which is usually called a bleaching agent) and dissolved with a silver ion-complexing agent which is called a fixing agent to remove the silver from the photographic element. As a result, only dye images remain in the photographic element. Practical developing processes are often accompanied by auxiliary steps for maintaining the photographic and physical quality of the images or for improving the storage stability of the images in addition to the above two basic steps (i.e., color development and desilvering). The auxiliary steps can include, for example, a hardening bath for preventing a light-sensitive layer from softening excessively in the processings, a stopping bath for effectively stopping the development, an image-stabilizing bath for stabilizing the images or a bath for removing a backing layer from the support. Color photographic light-sensitive elements employed in the color photography usually contain about 1 g to 15 g of silver per m.sup.2 of the element in the form of a silver halide. For example, reflection type image-forming elements, of which color papers are representative, contain 1 to 2.5 g of silver per m.sup.2 of the element, and photographic elements used in a camera, for example, color negative films or color reversal films, contain 3 to 9 g of silver per m.sup.2 of the element. This amount of silver is necessary for obtaining sufficient color density of images, but if the amount of silver can be reduced, many advantages such as conservation of silver resources and costs are achieved.
U.S. Pat. No. 3,674,490 discloses a color amplifying method can be used to reduce the amount of silver in photographic elements. In this method, metal images distributed in an image-wise manner act as a catalyst, whereby a peroxide such as hydrogen peroxide is activated to carry out an oxidation reaction and as the result, a dye is produced and, therefore, color images are formed. A very small amount of metal is used in the method because the metal images act as a catalyst. Accordingly, where the metal for forming images is silver, sufficient color density can be provided by using a smaller amount of a silver salt than that used in conventional gelatino-silver halide emulsions. In the method, the peroxide is used in an amplifying step following a developing step, of which color development is representative. However, this method has many disadvantages. For example, the peroxide is unstable in an aqueous solution, and the peroxide causes a marked degree of fog in the amplification.
U.S. Pat. Nos. 3,826,652, 3,834,907, 3,748,138 and 3,765,891 disclose other color amplifying methods can be used to reduce the amount of silver halide in photographic elements. In these methods, metal images which are distributed in an image-wise manner and act as catalysts cause an oxidation-reduction reaction with a Co (III) complex such as [Co(NH.sub.3).sub.6 ]Cl.sub.3 to occur to form dyes, whereby color images are formed. However, serious disadvantages also arise in these methods as well. That is, an amplifying solution or developer-amplifying solution containing the Co (III) complex is unstable with the passage of time. In other words, the Co (III) complex and a reducing agent undergo an oxidation-reduction reaction in the amplifying solution or developer-amplifying solution to form ammonia or amines, which generate fog. Therefore, image amplifying methods using a Co (III) complex have serious disadvantages.
Some proposals have been made to improve the above-described disadvantages in these methods. One improvement is disclosed in U.S. patent applications Ser. No. 307,891, filed Nov. 20, 1972 and Ser. No. 307,892, filed Nov. 20, 1972, corresponding to Belgian Patent Nos. 807,567 and 807,568, respectively, and U.S. Pat. Nos. 3,847,619 and 3,904,413, in which the Co (III) complex is incorporated into a silver halide light-sensitive element. However, the method has the disadvantage that sensitivity of silver halide is markedly decreased as a result.
The description in Research Disclosure 13,527 (No. 135, pages 19-20, July, 1975) is that tetrazolium salts can be added to an amplifying solution to capture a reducing agent, particularly a developing agent, incorporated into the amplifying solution. Further, the description in Research Disclosure 10,911 (No. 109, pages 25-28, May, 1973) is that active carbonyl compounds can be added to an amplifying solution to remove ammonia produced in the method. However, these two methods provide insufficient stability with the passage of time. It is believed that the reaction of the Co (III) complex and the developing agent should be prevented or controlled to eliminate the problems.
A method has been proposed in which an image amplification is carried out by using the Co (III) complex while the amount of amplifying solution used for replenishment is markedly increased to control the concentration of ammonia. But according to this method, large amounts of Co (III) complex which is expensive must be used, and pollution problems arise due to the waste Co (III) complex and Co (II) complex. As another improvement, including a washing step between the color developing step and the amplifying step has been proposed so that a developing agent is not carried over into the amplifying solution. This method results, however, in a less effective image amplification.
Two processes are known for image amplification using a Co (III) complex, that is, (1) color development.fwdarw.amplification.fwdarw.desilvering, and (2) simultaneous color development-amplficiation.fwdarw.desilvering. In process (1), one step (i.e., the amplifying step) is added to the steps in conventional color developing processes and fog occurs, while the concentration of the developing agent in the amplifying solution is kept lower than in process (2). Process (2) is more advantageous than process (1) in some points, but a reaction of Co (III) complex and color developing agent easily occurs because the Co (III) complex and the color developing agent are present in a high concentration, and the thus-produced ammonia or amine makes the process impractical.